Computer output microfilm machines are designed to place images on microfilm webs. The webs are stored wound on cylindrical cores. The exposed film webs are then developed and used for archival and other long-term, high-durability data storage requirements. Computer Output Microfilm machines (COM recorders) are used in two modes, cut fiche or roll mode. In each mode, it is desirable for the machine to have available information on the length of unexposed film remaining in the COM recorder. This information is used to assign various length jobs to COM recorders that have sufficient remaining unused film to complete the required printing length of the job. If the COM recorder runs out of film in the middle of a job, an operator must intervene, load more film, locate where the job terminated and restart the job. This is very labor intensive and costly to system operation. It is often difficult to locate where in the data stream the job terminated. In such circumstances, it may be necessary that the film be first processed to locate the last readable image and determine where the job must be restarted. One method used to eliminate this problem is to discard short (unknown) quantities of film in a supply cassette, which is a waste of film. The loading of film in a cassette capable of supplying information of the remaining unused film is of value to the user. The determination of the remaining length of film is conventionally accomplished with a computer algorithm that calculates remaining length from the thickness of the film web, the diameter of the core on which the film is mounted, and the number of fractional rotations of the core as the film is withdrawn. Even if a job is interrupted, or a partial roll of film inserted, by the time a few new images are laid down, the algorithm can calculate how much film is left.
Conventionally film has been provided in enclosures in the form of disposable cassettes. These cassettes incorporate a split plastic housing which encloses a film core rotating on an axle journaled on bearings within the housing. The film is restrained on the core by reel flanges that extend beyond the full wound diameter of the film. Fractional rotation detection is accomplished by multiple detectable elements positioned at or near the rim of the reel flanges. These detectable elements may be magnetic or conductive discrete elements that can be remotely detected by proximity or magnetic sensors (referred to hereinafter collectively as magnetic/proximity or detectors). The detectors are positioned outside the cassette and are capable of detecting the passage of each discrete element only through the non-conductive cassette materials. In part, for this reason, such cassettes have been made of plastic materials and are not designed for re-use. In another prior art configuration, optical detectors utilize notches in the rim to generate reciprocal pivoting movement in a pivoting element. The resulting movement is connected by a complex mechanical linkage to an external shutter that alternately blocks and passes light from a light source to an externally mounted photo-diode detector. In a still further configuration, a secondary external flange may be utilized, but requires a 1 for 1 relationship between the number of detectable elements and the number of detectable events for each revolution.
The use of disposable cassettes minimizes the opportunity to recycle any part of the mechanism and substantially increases the cost of the operation of the microfilming process. Conventional cassette design also necessitates the use of a complex loading process that includes winding the film web onto a core; attaching reel flanges; loading the reel into the first half of a split cassette housing and then securing the second half of the cassette housing to the first half. All of these operations must take place in darkroom conditions to avoid exposing the film.
The two types of cassettes require two different cassette molds, cassette loading operations and film inventory systems. Also, the internal detection mechanisms create problems in determining a "failure mode" in that the cassette must be destroyed to open to see if it is operating correctly.
It is, therefore, desirable to have a mechanism for storing, dispensing and measuring a length of microfilm web in a mechanism that is simple to load, reusable, and which is adaptable to existing machines that use either magnetic/proximity or optical detection.